This invention relates in general to alloy tubing and, more particularly, to high strength alloy tubing which is lined to retard coking and to a process for manufacturing such tubing.
Petroleum refineries produce a wide variety of products from petroleum feedstock, much of it by cracking, that is to say, by subjecting complex hydrocarbons to elevated temperatures in order to break them down into hydrocarbons of less complexity. Eythylene (C.sub.2 H.sub.4) is one product that is typically produced by cracking. To produce ethylene, a suitable feedstock is introduced into a cracking unit having multiple tubes formed from stainless steel, each with an inside diameter of about 4 inches and a length of up to 40 feet. The tubes are heated to between 1600.degree. F. and 1700.degree. F. As the feedstock passes through the tubes, it breaks down into ethylene and other less complex hydrocarbons.
The stainless steel of the tubing retains its strength at the elevated temperatures and further resists oxidation. In that sense, the stainless steel, which is typically 310 stainless steel, is well suited for the tubes of the cracking unit. But the nickel in the stainless steel, and to a measure the iron as well, serve as catalysts for the dehydrogenation of the hydrocarbons in the tubes. As a consequence, carbon deposits on the inside surfaces of the tubes. The initial layer of carbon promotes further deposits of amorphous carbon in the form of coke. The phenomenon is known as coking.
Coking reduces the inside diameter of the tubes, and that results in a greater pressure drop through the tubes. The coke also functions as thermal insulation where none is needed or wanted. As a consequence, the external temperatures of the tubes must be elevated to effect sufficient heat transfer. Moreover, where stainless steel serves as the alloy, the chromium within it exhibits an affinity for carbon and, indeed, some of it unites with the carbon deposits to form carbides. This not only reduces the capacity of the stainless steel to resist corrosion, but also renders it brittle and less capable of withstanding stresses.
Coking represents a serious problem for the operators of cracking units that produce ethylene. Periodically, the operator of a cracking unit must take the unit off line, disassemble it, and physically remove the coke from the interiors of the tubes in the unit. This time-consuming procedure sometimes must be repeated at six to ten day intervals.
An iron-aluminum alloy containing about 22% aluminum by weight resists coking quite well, and those who design cracker units know this. But the iron-aluminum alloy lacks strength at the elevated temperatures to which the tubing of cracker units is subjected, and hence, does not find use for such tubing.